What I spent the afternoon, evening, night and early morning on:
Only tested in ZSNES, but should work elsewhere.
And no, the rain is not the main feature. It's just used as a demonstration.
* Overworld is broken (graphically), which is why it's bypassed.
* The "old" thing appears when you defeat a boss, but you'll never see that.
* I had to remove the "floating points" effect when you pick stuff up etc. due to lack of space in OAM.
* 1-ups don't work, which seems to be a side effect of the above.
* I have a feeling that the Bowser battle will fail miserably, but I haven't tried it.
* The code is messed up beyond belief (due to a bad attempt to optimize it).
The rest of the problems you find are most likely caused by a crappy overworld bypass, not the main feature. (Yes, I actually execute a STP when you beat the level. Deal with it.)
Whether or not this will actually be released depends on a few things:
* Me fixing all the problems
* Me having enough time to complete it
* Me not being a douchebag after completing it